1. Field of the Invention
The invention relates in general to a constant flow fluid pump and in particular to means and method for reduction of pressure and flow pulsations in a fluid pump by selectively controlling the rotational speed of the pump driving motor at any one of a predetermined number of discrete rotational steps around the 360.degree. periphery of the driving motor rotation.
2. Description of the Related Art
There are many applications for analyzing blood and other fluids for which it is important to move the fluid to be examined at a uniform rate through testing/analyzing equipment, such as a flow cytometer. These fluids are usually driven by a constant pressure source. However the application of a constant pressure to a fluid may not result in a constant flow if the resistance to flow changes. For constant flow, the force pumps for driving these fluids are either of the diaphragm or reciprocating piston type of positive displacement pump that is actuated by an electric motor.
A problem with positive displacement pumps is that the rotary displacement of the electric motor must be converted to a linear displacement in order to activate the pump and thereby pump the fluid, i.e., both the diaphragm and the reciprocating piston are driven by a powered rod of some type that receives its linear motion by means of a reciprocating crankshaft. Whether it be a diaphragm pump or a reciprocating piston pump, the linear actuated rod must have its power converted from the rotary motion of the motor by means of a crankshaft/driving rod arrangement. It is well known that the output of a rotary motor driving a rod through a crankshaft arrangement, has a sinusoidal displacement output. The driving rod experiences displacement variations ranging from a minimum of zero at both top dead center and bottom dead center of its rotation through the crankshaft journal to a maximum displacement midway between top dead center and bottom dead center. It is also well understood in the Art that other parameters of the output pump also experience the same sinusoidal variation through the 360.degree. rotation of the driving motor through the crankshaft/driving rod arrangement. For example, it is well-known that the pressure and the flow output of both a diaphragm and a reciprocating piston pump consist of a half-rectified sine wave. If the pump is driving a purely resistive load, the pressure and flow will be in phase and have their maximum value when the crank of the pump is in the middle of its upstroke, at 90.degree. away from top dead center (TDC). After the pump passes TDC, the flow and pressure go to zero for a purely resistive load until the crank reaches bottom dead center (BDC).
Positive displacement pumps of the leadscrew drive type can provide a constant flow independent of resistance. However they must be refilled during the downstroke, during which time there is no output flow. Dual acting positive displacement pumps of the leadscrew drive type operate in tandem, so that as one pump is supplying fluid, the other pump is refilling. However these types of double acting pumps are expensive and complex.
A flow cytometer requires a pulseless flow of sheath fluid to obtain precise particle measurements. Present flow cytometers, in order to compensate for the pressure/flow variation described above, use one of two methods known in the Art to apply a pulseless flow of sheath fluid. The first is the use of a pressurized tank of sheath fluid that will even out the pulsations and the second is the use of a compliant member such as for example compressing a static volume of air through a flexible membrane. The problems with these two compensation methods is that the tank must have a very small height to prevent pressure variations from occurring as the tank empties and the tank must be sturdy enough to withstand pressure of 5-10 PSI, and that a constant pressure source doesn't provide a constant flow if the resistance to flow changes. Furthermore, the sheath fluid becomes saturated with air, which may be released as micro bubbles at the flow cell, causing the detection of false particles. The second method is equally problematic in the use of flow cytometry as well as other fluids analytical instruments in that the compliant member often is large and unwieldy and sometimes several compliant members are necessary to smooth pulsations in the flow of sheath fluid. Accordingly, it would be desirable to have a fluid pump driven by an electric motor through a crankshaft/driving rod arrangement that would have as close to a constant pressure and fluid output as possible through the 360.degree. rotational driving range of motion of the electric motor.